This invention relates to electrostatography and, more particularly, to a reproduction method and apparatus that employs transfers of toner images to and from intermediate transfer members.
In an electrophotographic process a photoconductive element is initially electrically charged. An electrostatic latent image is first formed by image-wise exposing the photoconductive element using an exposure source such as a laser scanner or an LED array. The latent image is developed into a visible image by bringing the electrostatic latent image into close proximity to a developer such as contained in a magnetic brush or other known type of development station. The developer can be what is typically referred to as a single component developer containing toner particles. Also, the developer can have two or more components with nonmarking magnetic carrier particles and marking non-magnetic toner particles. To produce color images, electrostatic latent images corresponding to the appropriate color are separately formed and developed. The resulting toner images are transferred to a receiver, such as a paper or a plastic sheet for example, preferably by using an electrostatic field to urge the toner in the direction of the receiver. The electrostatic field is commonly applied in one of several manners. For example, charge can be sprayed on to the back of a receiver using a corona device. However, it is frequently preferable to use an electrically biased transfer roller to apply the field, especially in instances where color images are to be produced.
It is often advantageous to transfer a toner image from an imaging member to an intermediate transfer member (ITM) in a first transfer nip, and from ITM to a receiver, e.g., paper, in a second transfer nip, rather than transferring directly from imaging member to a receiver. The second nip can be formed in a variety of ways, such as utilizing a biased transfer drum wrapped in a tensioned, electrically biasable transport web to which a receiver sheet is attached, or by using a dielectric transport web and an electrically biasable backup transfer roller. Color images are produced by developing and transferring toner images corresponding to the appropriate color separations. For example, to produce a full-color image, color separations having cyan, magenta, yellow and black toners typically are used. Color separation toner images are commonly transferred sequentially, in a registered manner, to an ITM and, subsequently, the complete color toner image is transferred from the ITM to a receiver in one step. Alternatively, individual color toner separation images can be transferred to separate ITMs, or to different sections of a single ITM, and then transferred in register to a receiver in multiple steps. Following a final toner transfer, a toner image on a receiver is permanently fixed using known processes such as thermal or radiant fusing.
In an electrographic process, a latent electrostatic image is created on a dielectric imaging member, e.g., by ionography or by electrified stylus or by other known means, and then toned. The resulting toner image, which can be a color separation image, can then be transferred to an ITM, and subsequently transferred to a receiver and fused as described above. An electrographic press can include a sequential series of modules, each module having an electrographic dielectric imaging member and an ITM for generating and transferring a color separation toner image to a receiver.
The nip is an engagement area of a roller under pressure with another member. This engagement area will result in some deformation. Pressure nips formed by rollers coated with elastomers are known to exhibit overdrive. Overdrive is the phenomena where the tangential speed of a roller within the nip engagement area is actually different than the tangential speed of the roller in an area not near the nip. Overdrive can be understood from a hypothetical consideration of a roller having an externally driven axle, frictionally driving a movable planar element having a nondeformable surface. If the external radius of the roller in an area not near the nip is R and the tangential speed of the roller in this area not near the nip is v0, then the surface velocity v of the distorted portion of the roller which is in nonslip contact with the planar surface is given by Equation 1.
v=xcexxcfx89Rxe2x80x83xe2x80x83Equation 1
where xcfx89 is the nominal angular rotational rate of the roller around its axis (radians per unit time) and where xcex is a peripheral speed ratio defined by
xcex=(v/v0).
The value of xcex is determined principally by the roller materials effective Poisson""s ratio and moduli, by the engagement and drag torque forces. The Poisson ratio, xcexd, of high polymers (those having a high molecular weight) approaches 0.5, and approaches zero for very soft polymeric foams. It has been shown in theoretical model computations by K. D. Stack, Nonlinear Finite Element Model of Axial Variation in Nip Mechanics with Application to Conical Rollers [Ph.D. Thesis, University of Rochester, Rochester, N.Y. (1995), FIGS. 5-6 and 5-7, pages 81 and 83] that for a special case of a rigid cylindrical roller coated by a layer of deformable material frictionally driving with no drag, a nondeformable moving planar element, the deformable material should have a value of Poisson""s ratio of about 0.3 in order to have negligible overdrive, i.e., xcex≈1. For values of Poisson""s ratio larger than about 0.3, the circumference of the roller (distorted by the nip) is greater than 2xcfx80R, producing overdrive of the planar element with respect to the roller, i.e., the surface speed within the nip of the coated roller (and hence that of the planar element) is greater than v0. For values of Poisson""s ratio smaller than about 0.3, the circumference of the roller is less than 2xcfx80R, producing underdrive of the planar element with respect to the roller, i.e., the surface speed is smaller than v0 within the nip. Conversely, if a nondeformable planar element frictionally drives, with negligible drag, a roller having xcexd less than about 0.3 and causes it to rotate, one can speak of overdrive of the roller with respect to the planar element because the surface speed of the driven roller far from the nip is faster than the speed of the planar element.
A foam or sponge can include a xe2x80x9cfeltedxe2x80x9d material, as is well known in the art. Felted foams can be made, for example, by compressing under heat, typically uniaxially, an elastomeric, previously made foam, followed by cooling it under compression and then removing the compressive load. Felted foams have anisotropic mechanical properties. For example, both the Young""s modulus and Poisson""s ratio of a felted foam material made by uniaxial compression will be different along the direction of compression that lies in a plane at right angles to the direction of compression. Moreover, Poisson""s ratio, which tends to be small for soft foams, can even take on negative values in felted foams or sponges.
Overdrive associated with two materials in contact in a pressure nip can result in squirming and undesirable stick-slip behavior. Such behavior can adversely affect image quality when more than one intermediate transfer member is used to make a color print in a color reproduction machine, e.g., by degrading the mutual registration of color separation images if the amount of overdrive produced by each ITM roller varies from roller to roller, or, by causing toner smear. Moreover, variations in overdrive from a given roller, sometimes referred to as xe2x80x9cdifferential overdrivexe2x80x9d, can occur axially or radially along the length of a transfer nip, such variations being produced, for example, by local changes in engagement, such as caused by runout, or by a lack of parallelism, or by variations of dimensions of the members forming a transfer nip.
An electrophotographic printing press can include a number of modules, one module for each color separation. Each module includes, for example, a photoconductive drum and an ITM in the form of a drum, e.g., forming a cyan toner image in module 1, a magenta image in module 2, a yellow image in module 3, and a black image in module 4. Each photoconductive drum has a primary charging station, an exposure station using a digital writer such as a laser scanner or LED array, a developing station, and a transfer station having a pressure nip where toner is electrostatically transferred to an appropriately electrically biased ITM drum, and a cleaning station. A receiver sheet, e.g., a paper sheet or a plastic transparency, is transported through successive modules, and the individual color separation toner images are successively transferred in register, from the successive ITMs of each module, to a receiver, e.g., paper, in successive transfer stations. Each transfer station, where a color toner image is transferred from an ITM to a receiver, includes a pressure nip formed with a suitably electrically biased transfer roller (backup roller) behind the receiver. A full-color toner image on a receiver is then sent to a fusing station where the toner is fused to the receiver. If, in practice, there is unregulated overdrive in the modules (the magnitude of overdrive can vary in degree from module to module) it is possible in principle to optimize registration (which is determined by the exact time of transfer from each ITM drum to receiver) by timing the writing of each latent image on each photoconductor drum, for example by sensing the location of an edge or a fiducial mark on a receiver sheet as it is transported from module to module. This can be costly, cumbersome and inconvenient. However, if there is also differential overdrive, then radial and axial variations of overdrive along a nip cannot be compensated for in such a manner, and the result can be unwanted losses of registration (and possible localized image smearing) in portions of a toned print. It should also be noted that significant drag forces result from the various contacting interfaces between drums and other elements in a module. These drag forces, which affect the amount of overdrive, can have time-dependent fluctuations which can also produce registration errors.
In making high quality images using small toners uses of compliant ITMs are disclosed, for example, in: Rimai et al., U.S. Pat. No. 5,084,735; Ng et al., U.S. Pat. No. 5,110,702; Zaretsky, U.S. Pat. No. 5,187,526; Rimai et al, U.S. Pat. No. 5,666,193; and Tombs et al., U.S. Pat. No. 5,689,787. The benefits of employing compliant ITMs are well known, especially as pertaining to their use with small toner particles. Small toner particles are defined as toner particles having a mean volume weighted diameter of between 2 micrometers and 9 micrometers, as determined by a suitable commercial particle sizing device such as a Coulter Multisizer. A prior art compliant ITM such as cited above in U.S. Pat. Nos. 5,084,735; 5,110,702; 5,187,526; 5,666,193; and 5,689,787 typically includes an elastomeric layer preferably between 1 mm and 25 mm in thickness, having a Young""s modulus between 1 MPa and 50 MPa and having an electrical resistivity between 106 ohm-cm and 1012 ohm-cm, preferably 107 ohm-cm to 109 ohm-cm. It is preferable that such a prior art compliant intermediate also include a relatively thin (0.1 mm to 20 mm thick) overcoat layer having a material whose Young""s modulus is greater than 100 MPa. Young""s modulus is determined on a macroscopic-size sample of the same material using standard techniques, such as by measuring the strain of the sample under an applied stress using a suitable commercial device such as an Instron Tensile Tester and extrapolating the slope of the curve back to zero applied stress.
The use of a common gear to provide equal rotational speeds (peripheral speeds) of an ITM and a PC is disclosed in U.S. Pat. No. 5,390,010 issued to Yamahata et al.,. There is a shortcoming within Yamahata et al.,. in that it does not solve problems due to overdrive, such as stick-slip motion that can results from elastic strain windup in one or both members forming a transfer nip, i.e., when (peripheral) speeds of members forming a nip are constrained to be equal and yet a nonequal speed is demanded by the overdrive physics.
M. Toshio et al., in U.S. Pat. No. 5,519,475, discloses a differential motion that produces a slip between a PC drum and an ITM in order to improve the transfer of toner. The embodiments disclosed employ gears to provide the differential motion. Specifically, the invention of M. Toshio et al., in U.S. Pat. No. 5,519,475 relates to a peripheral speed difference between PC and ITM in a range of 0.5% to 3.0%. See also Tanigawa et al., U.S. Pat. No. 5,438,398.
A fairly recent patent issued to S. Badesha et al., U.S. Pat. No. 5,576,818, and assigned to Xerox, discloses a multiple layer ITM having an electrically conductive substrate, a conformable and electrically resistive layer including of a first polymeric material, and a toner release layer having a second polymeric material. Also Toshio et al., in U.S. Pat. No. 5,519,475, disclose a multiple layer ITM having a conductive core, an intermediate resistance elastic layer, and a smooth, intermediate resistance outer layer which can include a different polymer.
Tanigawa et al., in U.S. Pat. No. 5,438,398, disclose an intermediate member having a metal pipe core covered by a single layer of an elastic material which can include a foam. The elastic material is 8 mm thick, contains dispersed carbon or zinc oxide or the like and has resistivity 105 ohm-cm-1011 ohm-cm. The elastic layer has 20xc2x0 to 40xc2x0 Asker C hardness. In a transfer nip for transfer of toner to a receiver sheet, the transfer roller is relatively much harder, made of a conductive core and a thin outer layer of a fluorinated resin having thickness 20 micrometers-100 micrometers. Also disclosed is a roller, which functions as an ITM for color imaging, whereby individual color toner images are successively electrostatically transferred to the ITM to build a full-color image on the ITM. Following this, a receiver sheet is passed through the same nip with the electric field direction in the nip reversed, thereby transferring the full-color toner image to the receiver, during which time the PC drum acts, in effect, as a transfer roller. The ITM has a metal core, an electrically conductive elastic layer having resistivity 103 ohm-cm-106 ohm-cm, and a thin outer layer having resistivity 107 ohm-cm-1011 ohm-cm. The electrically conductive elastic layer can be a foamed EPDM (ethylene propylene diene monomer) layer having dispersed carbon particles to give a resistivity of 103 ohm-cm, and the outer layer can be a low surface energy material, e.g., polyvinylidene fluoride containing tin oxide and having a resistivity of 109 ohm-cm. The hardness of the ITM roller is 35xc2x0 Asker C, i.e., the foam layer is quite soft and therefore significant overdrive is to be expected in a nip formed with the other, relatively hard, roller.
In order to achieve high image quality in respect to registration and lack of image smear in electrostatography, and in particular electrophotography, there is a need to provide an ITM in the form of a roller or a drum producing predictable and controllable overdrive behavior as a function of engagement in a nip of predetermined geometry. In particular, there is a need to provide an ITM for which overdrive is at most weakly dependent, and preferably independent, of engagement, not only in a first transfer nip with a relatively hard PC roller or drum, but also in a second transfer nip with a receiver backed by a relatively hard transfer roller. There is yet a further need to minimize overdrive sensitivity to engagement variations and other process noises, such as for example produced by runout such as produced by roller eccentricity or acentricity, or by a lack of parallelism or by variations of dimensions of the members or components of the members forming a transfer nip. There is a still further need to provide an ITM such that its overdrive (or underdrive) characteristics are insensitive to changes in drag, such as for example can be produced when a receiver sheet enters a second transfer nip.
The foregoing needs are satisfied by an ITM according to the invention that provides a method and apparatus of reducing both overdrive and differential overdrive associated with an intermediate transfer member roller of an electrostatographic color reproduction machine. The main benefit is a reduced sensitivity to fluctuations of engagement associated with roller runout, mounting tolerance errors, and the like. The invention provides an improved registration of color separation toner images, and an improved fidelity of reproduction with minimal distortion of an original or input image to be reproduced.
In accordance with the invention, there is provided an intermediate transfer roller for use in electrostatography including a method and apparatus for an intermediate transfer roller for use in electrostatography wherein a rigid cylindrical core member is provided then surrounded by an elastically deformable structure having a Poisson""s ratio in a range of 0.2 to 0.4. Preferably, the elastically deformable structure surrounding the core member has a conformable layer surrounding the core member, a compliant blanket layer surrounding and adhered to the conformable layer, and a thin hard outer layer formed on the compliant blanket layer. A thin flexible electrically conductive electrode layer adhered to the conformable layer is also discussed.